Natural Transition Process Research Articles

Regulating scramjet combustor mode transition using fuel distribution control

Dual-mode scramjets can operate efficiently over a range of flight speeds from moderate supersonic to hypersonic conditions. Depending on the fueling and flight conditions, the combustion mode operates in either a thermally-choked mode or a supersonic combustion mode. Direct-connect experiments were conducted using a laboratory-scale scramjet combustor with hydrogen as fuel, and its combustion mode transition behavior was characterized over various equivalence ratios. It was observed that the combustor became susceptible to combustion instability when mode transition was occurring naturally. To explore the possibility of actively triggering combustion mode transition while alleviating the combustion instability concerns, a new strategy of changing fuel injection distribution was formulated and a series of spatially distributed fuel injection experiments were conducted. The results showed that the critical amount of fueling for mode transition depends on the degree of fuel distribution. Subsequent experiments demonstrated that the combustion mode transition timing could be effectively controlled by scheduling spatial distribution of fuel injection. When fuel was injected at one location, most of heat release was concentrated near the cavity flame-holder, leading to thermal choking at a relatively low equivalence ratio. With distributed fuel injection, heat release became more evenly distributed across the expanding portion of the combustor, effectively delaying the mode transition to a higher equivalence ratio. Through the use of fast-acting solenoid valves, it was shown that changing fuel injection distribution could be used to trigger a timely combustor mode transition while holding the total fuel flow rate unchanged. When mode transition was actively triggered, the entire transition process occurred over a significantly shorter time scale compared to the natural mode transition process. The results indicate that combustion mode transition process could be controlled at the desired timing by actively scheduling fuel injection distribution, with reduced risks of encountering combustion instabilities while transitioning.

Reynolds-Number Effects on the Acoustic Disturbance Environment in a Mach 6 Nozzle

To understand the impact of the Reynolds number on the disturbance field inside a high-speed wind tunnel, we use direct numerical simulations to model turbulent boundary layers along the walls of a quasi-two-dimensional nozzle configuration. These simulations are performed at four different unit Reynolds numbers, ranging from to . The predominantly hydrodynamic fluctuations inside the boundary layer are nearly unaffected by the freestream forcing of the impinging acoustic radiation from the opposite wall. Within the nozzle-core region, the disturbance environment is found to be spatially uniform and solely acoustic in character. Comparisons with tunnel noise measurements are made using the numerical data. For the first time, comparisons of fluctuations in the same physical quantity (the streamwise mass flux) have been published, as opposed to earlier comparisons involving static-pressure fluctuations based on the simulations and pitot-pressure fluctuations recorded in the wind tunnel. The NASA 20 in. Mach 6 wind tunnel observations corroborate the predicted trend of decreased root-mean-square variations in pressure and mass flux as the Reynolds number rises. Additional details of the acoustic radiation field are quantified and should be useful toward a digital synthesis of the tunnel disturbance environment that would enable realistic simulations of the natural transition process.

Large Eddy Simulation of Transitional Separated Flow Over a Low Reynolds Number Cambered Airfoil

Abstract The accurate simulation of the aerodynamic behavior of low Reynolds number (Re) cambered airfoils requires the ability to capture the transitional separated boundary layer (BL) that occurs naturally on the surface of the airfoil. In this study, simulations are performed using a modern cambered airfoil designed for use in low Re applications, which are an advancement from previous studies using flat plate geometries or symmetric NACA airfoils. The cambered SD 7037 airfoil is simulated using wall-resolved large eddy simulation (LES) at a modest Re of 4.1×104 and at 1 deg, 5 deg, and 7 deg angles of attack (AOAs), with results validated against experimental data. Simulated predictions of pressure and skin friction coefficients clearly capture the correct location of the laminar separated bubble (LSB) which forms during the natural BL transition process. Sensitivity to elevated inflow turbulence is found to cause early BL reattachment at higher AOAs without impacting the location of BL separation. An integral BL analysis verifies the accuracy of the simulated velocity profiles against experimental values. The scale of horseshoe structures visualized in the transitional BL is larger in comparison to airfoil chord length than what is seen in previous simulations at Re of the order of 105, which highlights the importance of investigating cambered airfoils at a modest Re.

Techniques for Turbulence Tripping of Boundary Layers in RANS Simulations

The exact placement of the laminar–turbulent transition has a significant effect on relevant characteristics of the boundary layer and aerodynamics, such as drag, heat transfer and flow separation on e.g. wings and turbine blades. Tripping, which fixes the transition position, has been a valuable aid to wind-tunnel testing during the past 70 years, because it makes the transition independent of the local condition of the free-stream. Tripping helps to obey flow similarity for scaled models and serves as a passive control mechanism. Fundamental fluid-mechanics studies and many engineering developments are based on tripped cases. Therefore, it is essential for computational fluid dynamics (CFD) simulations to replicate the same forced transition, in spite of the advanced improvements in transition modelling. In the last decade, both direct numerical simulation (DNS) and large-eddy simulations (LES) include tripping methods in an effort to avoid the need for modeling the complex mechanisms associated with the natural transition process, which we would like to bring over to Reynolds-averaged Navier–Stokes (RANS) turbulence models. This paper investigates the implementation and performance of such a technique in RANS and specifically in the k-omega SST model. This study assesses RANS tripping with three alternatives: First, a recent approach of turbulence generation, denoted as turbulence-injection method (kI), is evaluated and investigated through different test cases; second, a predefined transition point is used in a traditional transition model (denoted as IM method); and third a novel formulation combining the two previous methods is proposed, denoted gamma -kI. The model is compared with DNS, LES and experimental data in a variety of test cases ranging from a turbulent boundary layer on a flat plate to the three-dimensional (3D) flow over a wing section. The desired tripping is achieved at the target location and the simulation results compare very well with the reference results. With the application of the novel model, the challenging transition region can be excluded from a simulation, and consequently more reliable results can be provided.

Influence of Nose-Tip Bluntness on Conical Boundary-Layer Instabilities at Mach 10

Hypersonic boundary-layer transition experiments are performed at the von Karman Institute for Fluid Dynamics in the low-enthalpy Longshot wind tunnel. Freestream Mach numbers are within the range , and freestream unit Reynolds numbers vary within the range . The model is a 7 deg half-angle cone, which is 800 mm long, with exchangeable nose tips for which the radii vary between 0.05 and 10 mm. The natural transition process is investigated using flush-mounted thermocouples, fast-response pressure sensors, and a laser-based schlieren flow visualization technique. The most amplified frequencies retrieved experimentally from wavelet analysis techniques compare excellently with linear stability theory results and confirm the inviscid nature of the instabilities observed along geometries with a small amount of nose-tip bluntness. The corresponding factors at the transition onset are inferred from the experimental location where transition is initiated. Schlieren images exhibit the typical rope shape of second-mode instabilities for the sharpest nose tips, but not along blunter geometries where instabilities peak outward of the boundary-layer edge. The different measurement techniques used show that the maximum amplitude of the second-mode waves is reached shortly after the location with the minimum wall heat flux. Schlieren images show that wave packets break down to turbulence near the middle of the transition region.

Characterization of a highly efficient chevron-shaped anti-contamination device

This paper is devoted to the characterization of an optimized chevron-shaped anti-contamination device (ACD). This device can prevent efficiently the propagation of turbulence from the fuselage along the attachment line (hypothetical streamline that spreads the flow going to suction side and the one going to pressure side) of swept wings and enables the development of a new laminar boundary layer downstream. More specifically, the aim is to prevent boundary-layer transition along the attachment line by a contamination process. This process is characterized by the typical Reynolds number $$\overline{R}$$ and the associated Poll’s criterion. Thus, ACD efficiency will be expressed in terms of $$\overline{R}$$ values. Some experiments performed on a new numerically optimized ACD have shown its ability to prevent leading-edge contamination up to $$\overline{R}$$ values close to the natural transition process of the laminar boundary layer along the attachment line. The corresponding stability analysis of the laminar boundary layer is made using the Gortler–Hammerlin stability approach. The study is completed with the different transition processes that can occur downstream the attachment line, around the airfoil, especially with crossflow analysis.

Numerical investigation of the development of three-dimensional wavepackets in a sharp cone boundary layer at Mach 6

AbstractDirect numerical simulations were performed to investigate wavepackets in a sharp cone boundary layer at Mach 6. In order to understand the natural transition process in hypersonic cone boundary layers, the flow was forced by a short-duration (localized) pulse. The pulse disturbance developed into a three-dimensional wavepacket, which consisted of a wide range of disturbance frequencies and wavenumbers. First, the linear development of the wavepacket was studied by forcing the flow with a low-amplitude pulse (0.001 % of the free-stream velocity). The dominant waves within the resulting wavepacket were identified as the second-mode axisymmetric disturbance waves. In addition, weaker first-mode oblique disturbance waves were also observed on the lateral sides of the wavepacket. In order to investigate the nonlinear transition regime, large-amplitude pulse disturbances (0.5 % of the free-stream velocity) were introduced. The response of the flow to the large-amplitude pulse disturbances indicated the presence of a fundamental resonance mechanism. Lower secondary peaks in the disturbance wave spectrum were identified at approximately half the frequency of the high-amplitude frequency band, suggesting the possibility of a subharmonic resonance mechanism. However, the spectrum also indicated that the fundamental resonance was much stronger than the subharmonic resonance. A secondary stability investigation using controlled disturbances confirmed that fundamental resonance is indeed a dominant mechanism compared to subharmonic resonance. Furthermore, strong peaks in the disturbance wave spectrum were also observed for low-azimuthal-wavenumber second-mode oblique waves, hinting at a possible oblique breakdown mechanism. Thus, the wavepacket simulations indicate that the second-mode fundamental resonance and oblique breakdown mechanisms are the strongest for the investigated flow. Hence, both mechanisms are likely to be relevant in the natural transition process for a cone boundary layer at Mach 6.

Randomness Representation in Turbulent Flows with Kolmogorov Complexity (In Mixing Layer)

Investigations on the definition and randomness of turbulence was reviewed at first. Then, the Kolmogorov complexity which measures the randomness were introduced. Numerical and graphic data in the mixing layer which was formed downstream of two-dimensional nozzle exit were compressed with the aid of a compression program. Approximated Kolmogorov complexity, AK, and normalized compression distance, NCD, were obtained. The AK indicated the regularity of the laminar flow and the randomness of the turbulent flow quantitatively. The NCD of the numerical value varied with data length. Between the same data, it approached zero, yet, on the other hand, between different data, it approached unity as the data length increased. The NCD of the numerical value in the natural transition process in the mixing layer increased monotonically downstream. Thus the NCD appears to be the measure of the transition process. In the natural transition process in the mixing layer, the AK of the numerical value and the NCD of the graphic data did not change monotonously in the downstream direction. Thus they contain some uncertainty for the measure of the transition process.

Randomness Representation in Turbulent Flows with Kolmogorov Complexity (In Mixing Layer)

Investigations on the definition and randomness of turbulence was reviewed at first. Then, the Kolmogorov complexity which measures the randomness were introduced. Numerical and graphic data in the mixing layer which was formed downstream of two-dimensional nozzle exit were compressed with the aid of a compression program. Approximated Kolmogorov complexity, AK, and normalized compression distance, NCD, were obtained. The AK indicated the regularity of the laminar flow and the randomness of the turbulent flow quantitatively. The NCD of the numerical value varied with data length. Between the same data, it approached zero, yet, on the other hand, between different data, it approached unity as the data length increased. The NCD of the numerical value in the natural transition process in the mixing layer increased monotonically downstream. Thus the NCD appears to be the measure of the transition process. In the natural transition process in the mixing layer, the AK of the numerical value and the NCD of the graphic data did not change monotonously in the downstream direction. Thus they contain some uncertainty for the measure of the transition process.

Effect of Local Periodic Disturbance on Mixing Layer Downstream of Two-Dimensional Jet (Spatial Structure and Quantitative Representation of Laminar-Turbulent Transition Process)

The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet, which issued from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. The oscillation frequency was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. In the oscillating state, the same phenomenon as in the natural transition process appeared more upstream. In the early stage within the nonlinear region, the growth of fluctuating velocity attenuated and the Reynolds shear stress component decreased. The decrease in their production rate due to the expansion of the mixing layer contributed to the attenuation and decrease. The probability of the streamwise and normal fluctuating velocity components taking the same sign, increased or decreased in accordance with the increase and decrease of the Reynolds shear stress component. The randomness factor, which had been proposed by Sato, appeared to be a reasonable indicator of the present transition process, especially in the process in which the periodic velocity fluctuation became irregular. However, this factor certainly indicated the same value at two streamwise positions.

Mechanically Amplified Piezoelectric Microactuators for Laminar-Turbulent Transition Control on Airfoils

Drag reduction on airfoils using arrays of micro-actuators is one application of so-called Aero-MEMS. These microactuators interact with TS instabilities (Tollmien-Schlichting waves) inside a transitional boundary layer by superimposing artificially generated counterwaves in order to delay the transition process. These actuators need to exhibit a relatively large stroke at relatively high operational frequencies when operated at high Mach numbers. For this purpose, a novel micromachined mechanical amplification unit for increasing the stroke of piezoelectric microactuators up to high frequencies is proposed. The mechanical lever is provided by a sliced nickel titanium membrane. In this work, the actuator is explained in detail and wind tunnel experiments are carried out to investigate the effect of this mechanically amplified piezoelectric microactuator on thin transitional boundary layers. The experiments have been carried out in the transonic wind tunnel facility of the Berlin University of Technology on an unswept test wing with an NACA 0004 leading edge. The effectiveness of the actuator for flow control applications is determined in an open-loop setup consisting of one actuator having a relevant spanwise extension and a microstructured hot film sensor array located downstream. The aerodynamic results at Mach 0.33 are presented and discussed. It is shown that the actuator influences TS wave specific frequencies between 2.5 kHz and 7.4 kHz. The actuator amplitude is large enough to influence a transitional boundary layer significantly without bypassing the natural transition process which makes this type of micromachined actuator a candidate for high speed TS-control.

"I should live and finish it": a qualitative inquiry into Turkish women's menopause experience.

BackgroundWhile bio-medically, menopause could be treated as an illness, from a psychosocial and cultural perspective it could be seen as a "natural" process without requiring medication unless severe symptoms are present.Our objective is to explore the perceptions of Turkish women regarding menopause and Hormone Therapy (HT) to provide health care workers with an insight into the needs and expectations of postmenopausal women.MethodsA qualitative inquiry through semi-structured, in-depth interviews was used to explore the study questions. We used a purposive sampling and included an equal number of participants who complained about the climacteric symptoms and those who visited the outpatient department for a problem other than climacteric symptoms but when asked declared that they had been experiencing climacteric symptoms. The interview questions focused on two areas; 1) knowledge, experiences, attitudes and beliefs about menopause and; 2) menopause-related experiences and ways to cope with menopause and perception of HT.ResultsMost of the participants defined menopause as a natural transition process that one should go through. Cleanliness, maturity, comfort of not having a period and positive changes in health behaviour were the concepts positively attributed to menopause, whereas hot flushes, getting old and difficulties in relationships were the negatives. Osteoporosis was an important concern for most of the participants. To deal with the symptoms, the non-pharmacological options were mostly favoured.ConclusionTo our knowledge, this is the first qualitative study which focuses on Turkish women's menopausal experiences. Menopause was thought to be a natural process which was characterised by positive and negative features. Understanding these features and their implications in these women's lives may assist healthcare workers in helping their clients with menopause.

Study of Transition Mechanism in a Wake Behind an Airfoil with a Small Angle of Attack by Using a Towing Wind Tunnel

This paper describes an experimental investigation of the transition mechanism of a wake generated behind a thin airfoil with a small angle of attack in a towing wind tunnel. A linear stability analysis shows that the wake is characterized by a change in the instability from locally absolute to locally convective at a downstream location (30 mm) of the airfoil trailing-edge. When the airfoil is towed in the tunnel, boundary layers develop on the upper/lower airfoil surfaces with different thicknesses. Since the asymmetric wake is generated, starting vortices of a single row are observed first in the wake, which is different from the wake Karman vortex street. The experimental result shows that time-harmonic fluctuations of the starting vortex sustain in the natural transition process due to a self sustained resonance in the absolutely unstable region behind the trailing edge. The wake profile in the saturation steady state yields the vortex street structure, where the fluctuation frequency is defined as the fundamental unstable mode by the final saturation steady state. The growth of the fundamental unstable mode in the convectively unstable region suppresses the high frequency fluctuations associated with the staring vortex generation. On the other hand, low-frequency fluctuations in the quasi-steady state sustaining in the saturation state grow gradually during the vortex street formation, which lead to the vortex street deformation downstream.

Study of the Laminar-turbulent Transition of a Wake Behind an Airfoil by Using a Towing Wind Tunnel

This paper describes an experimental investigation of the natural transition mechanism of a wake generated behind a thin airfoil in a towing wind tunnel. A linear stability analysis shows that the wake is characterized by a change in the instability from locally absolute to locally convective at a downstream location of the airfoil trailing-edge. When the airfoil is towed in the tunnel, starting vortices are observed first in the wake. The experimental result shows that the feature of startig vortex sustains in the natural transition process due to a self sustained resonance in the absolute unstable region behind the trailing edge. The wake profile in the saturation steady state yields the vortex street structure, where the fluctuation frequency is defined as the fundamental unstable mode by the final saturation steady state. The growth of the fundamental unstable mode in the convectively unstable region suppresses the high frequency fluctuations associated with the staring vortex generation. On the other hand, low-frequency fluctuations in the quasisteady state sustaining in the saturation state grow gradually during the vortex street formation, which lead to the vortex street deformation downstream.

1453 絶対不安定な後流の乱流遷移に関する実験的研究

This paper describes an experimental investigating of natural transition mechanism of a wake forming behind a thin aerofoil in a towing wind tunnel. A linear stability analysis shows that an absolute instability pocket occurs downstream of the aerofoil trailing-edge. In the process of natural transition, the slight disturbances existing in the flow grow up selectively, and create the structures of vortex street in the wake. When the aerofoil is towed in the tunnel, starting vortices are generated in the wake. The experimental result shows that the feature of starting vortex affects the natural transition process due to the existence of absolute unstable region.